SHIYUAN LI et.al: A NEW METHOD TO INVESTIGATE RHEOLOGY OF ZECHTEIN SALT AT REST THROUGH … A New Method to Investigate Rheology of Zechstein Salt at Rest through Inverting the Sinking of Stringer Fragments Shiyuan Li*, Yuekun Xing School of Petroleum engineering, China University of Petroleum, Beijing, 102249, P.R. China Abstract - There is a heated debate about the long-term rheology of salt at rest. As many researches show, pressure solution creep and dislocation creep mechanisms dominate mechanical behavior of salt. We summarize the database of researches of salt rheology for the first time. A new method is used to evaluate salt rheology through inverting the gravitational sinking of anhydrite inclusions (also called ‘stringers’) embedded in Zechstein salt at rest during the Tertiary. Seismic data can be used to compare with numerical model of sinking stringer. Results show that under the control of dislocation creep, the fragments has almost no movement but under the control of pressure solution creep, the fragments will sink obviously through the salt section during a few Ma (some fragments can sink hundreds of meters and touch the presalt). In conclusion, through inverting the sinking of stringer fragments the long-term Zechstein salt rheology can be investigated and dislocation creep dominates its deformation during Tertiary. Keywords - long-term rheology, pressure solution creep, dislocation creep, gravitational sinking of stringer, Zechstein salt. E-mail: [email protected] I. INTRODUCTION of the salt, A is a material dependent parameter, Q is the 0 specific activation energy while R is the gas constant Rocksalt and salt structure have strong relationship with (R=8.314Jmol-1k-1) and T is the temperature. Moreover, petro-geology and petroleum engineering. Since 1980s in solution-precipitation creep (pressure solution) is also an last century, a lot of geologists have studied on salt important deformation mechanism in rocksalt and it is structure due to oil and gas exploration. Many geologists, shown in the previous researches [4, 28-29].The mechanism researchers of rock mechanics and petroleum or mining solution-precipitation creep is described following as a engineers concentrated on the studies of creep property of Newtonian flow law: rocksalt. Chinese researchers have widely applied the achievement of the study on creep deformation and damage nnQ 13 PS BB() exp()( ) (2) on the establishment of salt carven and drilling engineering 0 RT TDm of salt-gypsum layer. In this paper, we first build the and the strain rate is dependent on the strain size D, Δσ = database of salt rheology mechanism through the methods σ1-σ3 is the differential stress and B= B0 exp(-Q/RT)/TDm of laboratory experiment, microstructure observation and is the viscosity of the salt, B0 is a material dependent displacement analysis in active salt tectonics. We parameter, Q is the specific activation energy while R is the summarized two dominant deformation mechanisms. gas constant (R=8.314Jmol-1k-1) and T is the temperature. The order m influences the strain rate dependence on the In the last 30 years, some researches have been grain size. Finally, the total strain rate are the sum of the operated on the deformation mechanism of rocksalt at a two strain rates: range of temperatures (20-200°C). Dislocation creep [1-17] mechanisms are widely investigated . In the salt mining DC PS industry, salt deformation is controlled by dislocation creep mechanism which is shown laboratory experiments [18-27]. It is clearly demonstrated in Figure. (1) that relation The steady-state strain rate follows power-law creep (non- between strain rate and differential stress in these Newtonian) equation: experiments are controlled by power-law equation with the power order 5 which is relevant to dislocation creep when nnQ DC AA() 013 exp()() the salt deforms at 30°C to 200°C [26, 30-34]. At each RT temperature value, the strain rate can increase 2 or 3 orders has been used where έDC is the strain rate, Δσ =σ1-σ3 is the of magnitude at the same differential stress. The data in differential stress and A=A0exp(-Q/RT) is the viscosity Figure 2 shows that the salt rheology is dependent on three important factors for wet halite deformation, the grain size, *Address correspondence to this author at the School of Petroleum water content and temperature. engineering, China University of Petroleum, Beijing, 102249, P.R. China; Tel: +86 10 89739073; Fax: +86 10 89734150; DOI 10.5013/IJSSST.a.17.46.38 38.1 ISSN: 1473-804x online, 1473-8031 print SHIYUAN LI et.al: A NEW METHOD TO INVESTIGATE RHEOLOGY OF ZECHTEIN SALT AT REST THROUGH … Figure 1. The relation between strain rate and differential stress of coarse grain at 20-200°C Figure 2. The relation between strain rate and differential stress of wet fine and coarse grain at 20-200°C Due to time scale constraint in the experiment, it is density difference between the salt and anhydrite inclusion, rather difficult to realize the deformation process with the the stringers can sink towards the bottom of the salt bodies. strain rate below 10-9s-1, in figures we put two creep laws as One major issue contributing to the complexity of the not only comparison but also an extrapolation of the data problem are the rheological differences between the salt, the which were observed in the experiments. The rheology of embedded carbonates and the sediments overlaying the salt slow deformation is controversial. body. A few numerical studies about the salt deformation of salt structures has been implemented [35-38]. In salt II. SIMULATION ON METHODS AND MODELS structures, there are carbonate or anhydrite inclusions In nature, large anhydrite inclusions, called stringers, embedded and the movement and displacement of are embedded in salt structure. In our study, a very inclusions are researched [38-42]. Since there is obvious DOI 10.5013/IJSSST.a.17.46.38 38.2 ISSN: 1473-804x online, 1473-8031 print SHIYUAN LI et.al: A NEW METHOD TO INVESTIGATE RHEOLOGY OF ZECHTEIN SALT AT REST THROUGH … Figure 3. (a) East–West seismic profile through the study area with the stringer interpolation surface A (black, dashed line) and the smoothed surface B (white, dashed line) and (b) its interpretation figure. In (b), blue represents the Z2–Z4 salt section between the pre-salt, brown the Basal Zechstein, Rotliegend and Carboniferous and yellow the sedimentary overburden. Note the differences in Z3 stringer geometry west and east of the central salt wall related to structural features of the Top Salt TABLE I. MATERIAL PARAMETERS FOR ANHYDRITE AND ROCKSALT (FOLLOWING THE TWO DIFFERENT DEFORMATION MECHANISMS AT 50° C (I.E. LOWER LIMIT OF THE MODEL)) Salt A: Pressure solution creep Salt B: dislocation creep (Non- Anhydrite (Newtonian flow) newtonian flow) blocks -4 -1 -1 -9 -5 -1 A0 4.70×10 MPa s 1.802×10 MPa s - Q [J/mol] 24530 53920 - R [J/mol] 8.314 8.314 - m,n 3/1 0/ 5 - ρ/density [kg/m3] 2200 2200 2900 E/Youngs Modulus [GPa] 10 10 40 ν/ Poisson's ratio 0.4 0.4 0.3 Reference [28] [26] [46] On average the Zechstein salt thickness above the stringer is important process is anhydrite stringer (or stringer less than fragments) can sink at a certain speed until the bottom of salt body due to the density contract between salt and that below (Figure. (3)), and hence the stringer mostly anhydrite rock. We use commercial software ABAQUS to occurs close to or at the Top Salt. The study area is located build numerical model and we model gravitational sinking in the Dutch offshore (area introduced as ‘western off- of stringer in Zechstein salt with two different dominant shore’[43]) on the Cleaver Bank High. These basins were laws after salt tectonic stopped. The numerical results can inverted in the Cretaceous and Tertiary [44]. All the stringer be used to compare with seismic data. The seismic data we fragments embedded in the Zechstein salt should reach the use is provided [43, 44]. The study by van Gent et al. bottom of salt body if the stringer can sink rapidly due to (2011) has already shown that the on average 40–50 m gravitational loading [43, 44]. thick Z3 stringer has a complex structure dominated by It is an effective method to investigate salt rheology boudinage and folding, frequently offset vertically over through inverting stringer sinking in salt [45]. The results of more than half of the total Zechstein thickness (Figure. (3)). movement and stress around stringer can be compared to DOI 10.5013/IJSSST.a.17.46.38 38.3 ISSN: 1473-804x online, 1473-8031 print SHIYUAN LI et.al: A NEW METHOD TO INVESTIGATE RHEOLOGY OF ZECHTEIN SALT AT REST THROUGH … the seismic data. We choose typical value of mechanical properties for salt and embedded stringer in Table 1. We built a simplified model of the Zechstein salt section with physically isolated stringer fragments of 50m thickness A initial model and 100, 200, 300m width embedded in a 10km width and 500m thickness salt section similar to Figure. (3) (observed by the research [44]). The model is 2D plane strain.The boundary conditions are zero displacement perpendicular to the boundary at the bottom and at the sides. Here we only model the time period after tectonic setup, the Zechstein B -60Ma salt at rest in Tertiary. Modeling the breakup of stringers is not necessary in the research [38].This post-tectonic setup is simulated about a time span of 60 Ma to simulate the Zechstein salt at rest in the Tertiary. Table 2 summarizes the geometric parameters used for the stringer fragments, the salt section and the time duration of experimental C -30Ma phases.